The present invention relates to rolls for use in calenders and analogous machines. More specifically, the invention relates to improvements in rolls of the type wherein a hollow cylindrical shell is rotatable about a stationary carrier which supports one or more bearing elements and one or more hydraulic pressure generators in the form of cylinder and piston units serving to urge the bearing element(s) against the internal surface of the shell. Still more particularly, the invention relates to improvements in rolls of the type wherein each bearing element has an external surface provided with one or more recesses or pockets communicating with the cylinder chamber or chambers of the associated pressure generating device(s) and wherein the pressure generating device or devices are connected with one or more sources of pressurized hydraulic fluid.
German Offenlegungsschrift No. 29 42 002 discloses a roll wherein the shell is rotatable about a fixedly mounted carrier which supports an axially extending row of bearing elements each having pockets machined into that surface which is adjacent to the internal surface of the shell. Each bearing element is acted upon by at least two discrete pressure generating devices in the form of cylinder and piston units. The piston of each such unit is secured to the carrier and extends into a cylinder chamber which is provided in the corresponding bearing element and communicates with a pocket in the external surface of such bearing element. The pistons are surrounded by seals which fit snugly into the respective cylinder chambers. The connections between the cylinder chambers and the pockets in the external surfaces of the bearing elements constitute or contain flow restricting elements. The individual cylinder and piston units for a given bearing element and/or the cylinder and piston units for different bearing elements can be connected to separate sources of pressurized fluid. This enables the shell to resist flexing or bending stresses as well as to compensate for transversely acting forces which develop when the roll is in use in a calender or the like and cooperates with one or more similar or otherwise constructed rolls to treat a running web of paper, fabric or the like.
When a hydrostatically supported shell of the just outlined character exhibits certain defects which are attributable to the design of the roll and/or which develop while the roll is in use, the shell is subjected to abrupt and pronounced stresses and often tends to vibrate. Initial (manufacturing) defects can include improper balancing of the shell, deviations from acceptable tolerances and/or deviations from optimum mounting in the frame of a calender or another machine, as considered in the axial direction of the roll. Defects which develop in actual use can include thermally induced expansion or contraction of the shell and/or other constituents of the roll. The problems which arise as a result of vibrations and/or development of abrupt and pronounced stresses are particularly serious when the roll forms part of a system of rolls which treat running webs of paper, fabric or the like by the application of pressure in that the roll cooperates with one or more additional rolls to impart to the corresponding side or sides of the running web a desired finish, texture, design and/or other characteristics. Defects in the configuration, dimensioning and/or mounting of one or more rolls can influence the operation of the remaining roll or rolls. Also, improper distribution of mass in the material which is being treated can also cause the development of undesirable stresses which can affect the useful life and/or the mode of operation of the roll or rolls, i.e., the quality of treatment which such roll or rolls impart to the processed material. The stresses upon the roll or rolls can be sufficiently pronounced to cause the establishment of actual contact between the external surface(s) of the bearing element(s) and the internal surface of the shell. Moreover, uncontrolled and pronounced stressing of the bearing elements can result in extensive wear upon and/or rapid destruction of the aforementioned seals which surround the pistons of the pressure generating devices for the bearing elements. This, in turn, can result in complete breakdown of the hydrostatic bearing system. Still further, improper functioning of the hydrostatic bearing system can adversely influence the finish of the external surfaces of the shells and hence the treatment of the running web or webs which are contacted by such external surfaces.